pthread-stubs: rename to xlib_libpthread-stubs

pthread-stubs: rename to xlib_libpthread-stubs

In upstream X.org releases, pthread-stubs is named libpthread-stubs,
so for consistency reasons, we rename the Buildroot package
accordingly. Also, while we're at it, we add a xlib_ prefix to the
package to match other X.org libraries in Buildroot.

The necessary Config.in.legacy code is added to ensure that users
having .config files using the old configuration option name get a
warning.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>